There's a few things you can try for dealing with a dead battery. Two are quick and one is slow.
1. Try a bump start. Some people recommend using second gear. Anyway, turn the ignition on, make sure the kill switch is set to run, etc., and put the bike in gear. With your buddy on his bike and holding the clutch in, you push the bike to get some speed going (down a gentle grade is nice) then your buddy can release the clutch gently, which will cause the engine to crank - and maybe start. Your buddy needs to be ready to pull the clutch in again whether the engine starts or not.
Bump starting a bike can be easy or tricky, depending on many factors, including your own 'have a clue-ness'. It is possible to bump start a bike by yourself. Be careful or the bike will end up on its side (or worse) and you and your buddy will be sad. Hopefully you have someplace other than a busy road to do this.
2. Try jump starting it. Use jumper cables and your car / truck. Standard sequence for connecting, as if it's a car. With the car / truck running, try starting the motorcycle. Don't crank too long.
In either case, if the bike starts, let it run a while in order to charge the battery some. Ideally, your buddy can go for a 20 - 30 minute ride to build up at least a little charge in it. Rev's above 3k or so are better for charging than below.
3. Buy a battery tender (or charger) and charge the battery. This technique takes the most time and some investment of money but is quite reliable and easier.
Letting a battery run completely out of charge is a good way to shorten its life, if not kill it altogether. Even if you can revive the old one, your buddy should see about getting a new battery. Better safe than sorry, I say. Being stranded away from home due to a bad battery ruins my day.
Peace.
